Transcriptome Analysis of Salt-Sensitive and Tolerant Genotypes Reveals Salt-Tolerance Metabolic Pathways in Sugar Beet

Abstract

Soil salinization is a common environmental problem that seriously affects the yield and quality of crops. Sugar beet (Beta vulgaris L.), one of the main sugar crops in the world, shows a strong tolerance to salt stress. To decipher the molecular mechanism of sugar beet under salt stress, we conducted transcriptomic analyses of two contrasting sugar beet genotypes. To the best of our knowledge, this is the first comparison of salt-response transcriptomes in sugar beet with contrasting genotypes. Compared to the salt-sensitive cultivar (S710), the salt-tolerant one (T710MU) showed better growth and exhibited a higher chlorophyll content, higher antioxidant enzyme activity, and increased levels of osmotic adjustment molecules. Based on a high-throughput experimental system, 1714 differentially expressed genes were identified in the leaves of the salt-sensitive genotype, and 2912 in the salt-tolerant one. Many of the differentially expressed genes were involved in stress and defense responses, metabolic processes, signal transduction, transport processes, and cell wall synthesis. Moreover, expression patterns of several genes differed between the two cultivars in response to salt stress, and several key pathways involved in determining the salt tolerance of sugar beet, were identified. Our results revealed the mechanism of salt tolerance in sugar beet and provided potential metabolic pathways and gene markers for growing salt-tolerant cultivars.

Keywords: differentially expressed gene; physiological analysis; salt stress; sugar beet; transcriptomic analysis.

Figure 1

Morphological and physiological changes in sugar beet (S710 and T710MU) under control (0 mM NaCl) and salt stress (280 mM NaCl) conditions. Seven-day-old seedlings were treated with 280 mM NaCl for 15 days. Growth performance (a), fresh weight (b) and dry weight (c) were measured. Different letters indicate significantly different at p < 0.05. Three biological replicates were performed.

Figure 2

Effects of salt stress on photosynthesis-related parameters in the leaves of two sugar beet genotypes. chlorophyll content (a), net photosynthetic rate (b) and stomatal conductance (c) were measured. Different letters indicate significantly different at p < 0.05. Three biological replicates were performed.

Figure 3

Effects of salt stress on antioxidant enzyme activity in the leaves of two sugar beet genotypes. The malondialdehyde (MDA) content (a), relative conductivity (b), superoxide dismutase (SOD) (c), catalase (CAT) (d), peroxidase (POD) (e) and ascorbic acid peroxidase (APX) (f) activity in sugar beet treated with 0 mM and 280 mM NaCl. Different letters indicate significantly different at p < 0.05. Three biological replicates were performed.

Figure 4

Effects of salt stress on the levels of osmotic adjustment substances in the leaves of two sugar beet genotypes. The levels of betaine (a), proline (b) and free amino acids (c) in sugar beet treated with 0 mM and 280 mM NaCl. Different letters indicate significantly different at p < 0.05. Three biological replicates were performed.

Figure 5

Volcano-plots and functional classification analysis of differentially expressed genes (DEG_S) in the leaves of two sugar beet cultivars under salt stress. DEGs in the leaves of cultivars S710 (a,c) and T710MU (b,d). Each dot in panel a and b represents a specific gene or transcript, the red dots represent genes that were significantly up-regulated, the blue dots genes that were significantly down-regulated, and the black dots non-significantly changed DEGs.

Figure 6

Top 10 KEGG pathways in the leaves of sugar beet cultivars S710 and T710MU under salt stress. KEGG enrichment pathways in S710 (a) and T710MU (b) under salt stress.

Figure 7

Relative expression of genes involved in salt stress in two sugar beet cultivars examined using RT-qPCR. The expression of UDP-D-xylose synthase 2 (AXS2) (a), Galacturonosyltransferase 10 (GAUT10) (b), Calcineurin B-like protein 2 (CBL2) (c), Phosphoserine aminotransferase (SerC) (d), Choline monooxygenase (CMO) (e) and Primary amine oxidase (PAO) (f) were detected. Different letters indicate significantly different at p < 0.05. Three biological replicates were performed.

Figure 8

Schematic presentation of the potential mechanism of salt stress tolerance in T710MU tolerant sugar beet cultivar. The red highlighted genes indicate only up-regulation in salt-tolerant T710MU.